This invention relates generally to feedback control for turbine driven high power alternating current synchronous generator systems.
This invention relates more specifically to generator systems in which oscillations in the mechanical portion thereof due to subsynchronous resonance is a problem. Electrical generators are often driven from power sources such as steam turbines. Multiple rotating masses are usually present on a single shaft thereof. As an example, the rotating mass in one or more of the turbines, the rotating mass of the exciter for the AC generator and the rotating mass of the generator rotor may all be present. It has been found that because of the mechanical properties of the various parts of the rotating system, torsional oscillation will result involving the rotating masses. The torsional oscillation may result from disturbances internal to the generator or from disturbances external to the generator along the electrical system supplied thereby. If these torsional oscillations become large and undamped, they can cause shaft breakage. It has been found in certain applications that such undamped torsional oscillation may exist because of the electrical properties of the electrical system supplied by the AC synchronous generator. As an example, when long transmission lines are required for the delivery of energy to a grid network or load the inductive effect of the long transmission lines may require series capacitive compensation. However, the effect of the compensating capacitors being connected in series circuit relationship with the inductance tends to cause certain resonant frequencies to exist in the electrical network. The electrical effect of these resonant frequencies may be fed by the magnetic coupling between the stator and the rotor of the synchronous generator to the mechanically rotating shaft. If the previously described electrical resonance cooperates with a critical mechanical resonance between the masses of the generating system, reinforced, undamped torsional oscillation will occur which may be of a sufficient magnitude to break the shaft. Apparatus and method for solving this problem has been proposed in the past in U.S. Pat. No. 3,662,251, issued May 9, 1972 to O. J. M. Smith, entitled "Method and System for Measuring Acceleration and Velocity". This latter patent is related to the sensing of oscillations rather than providing a method for directly correcting for the effect of the oscillation. A similar measuring method and apparatus is described in U.S. Pat. No. 3,662,252, issued May 9, 1972 to O. J. M. Smith, entitled "Tachometer and Method of Attaining a Signal Indicative of Alternating Shaft Speed". Other U.S. patents propose apparatus for stabilizing low frequency power oscillation by measuring some parameter of a system and operating on the field excitation of the generator for corrective action. Examples of these patents are: U.S. Pat. No. 3,477,014, issued to A. L. Blyth on Nov. 4, 1969 and entitled "Electrical Control System with Stabilizing Control Means", U.S. Pat. No. 2,981,882, issued Apr. 25, 1961 to M. Rosenblatt, entitled "Stabilizing Circuit for Dynamoelectric Machines" and U.S. Pat. No. 3,656,048, issued Apr. 11, 1972 to A. W. Hauf, entitled "Non-Linear Exciter Controller for Power System Damping". Patents which are of interest for showing field control in electrical apparatus are: U.S. Pat No. 2,854,617, issued Sept. 30, 1958 to L. J. Johnson and entitled "Frequency Control Apparatus for Alternators" and U.S. Pat. No. 3,474,323, issued Oct. 21, 1969 to L. A. Kilgore et al, entitled "Electrical Control Systems with Stabilizing Control Means". Still other patents which may be of interest generally in this area are: U.S. Pat. No. 3,119,934, issued Jan. 28, 1964 to R. H. Lee, entitled "Generator Control Means" and U.S. Pat. No. 3,167,702, issued Jan. 26, 1965 to A. Schonung et al, entitled "Vibration Damping Arrangement for Rotary Field System of Excitation Regulation Synchronous Machines". Another patent which is related to frequency control by mechanical means is U.S. Pat. No. 2,767,367, issued Oct. 16, 1966, to L. Blank and entitled "Generator Frequency Control by Electrical Braking". All of the aforementioned patents are background patents for U.S. Pat. No. 3,999,115, issued Dec. 21, 1976 to South et al and entitled "Dynamic Stabilizer for Synchronous Machines Having Torsional Oscillations and Method". Although it is submitted that the latter patent represents a significant improvement in the state of the art for compensating for the effect of mechanical torsional oscillation as it is affected by electrical power line resonance where the compensation takes place in the electrical system per se, certain disadvantages lie in the fact that the compensating apparatus for the latter patent is essentially series connected in the transmission system of the compensator. Furthermore, the apparatus of the latter mentioned patent requires a frequency conversion. The frequency conversion is related to the frequency of the line resonance rather than the frequency of the torsional oscillation resonance. Finally, another disadvantage lies in the fact that in order to take the device out of service it must be short-circuited. The previously described cross-related application teaches a device which is essentially parallel connected rather than series connected. With the latter device thyristor switched inductors are connected between phase conductors of the AC system in the vicinity of the terminals of the turbine generator. Appropriate sensing and feedback control is provided between the electrical lines and the thyristor switched inductors to modulate the conduction angle of the thyristor switch about a relatively high bias conduction angle of approximately 135.degree.. The modulation of the conduction angle produces the effect of modulating the inductance connected between the lines and it is this modulated inductance and the reactive current which is produced thereby which stabilizes the electromechanical generator system. The conduction angle of 135.degree. is provided to give a dynamic range to the system for stabilizing the system. When the system is exposed to the effects of load disturbances the amount of power consumed in the stabilizer when the 135.degree. quiescence conduction angle is utilized is easily justified. However, when there are no significant load disturbances the amount of standby power used at the 135.degree. conductive angle tends to be relatively high compared with the power which is needed to stabilize the system from internal disturbances. It would be advantageous therefore if a stabilizer could be found which had the dynamic range to easily stabilize the electromechanical system from the effects of externally introduced disturbances and yet utilized relatively little power when the external disturbances were not present.